1. The Field of the Invention
The present invention relates generally to methods of forming coated articles. In particular, the present invention relates to methods for producing coated articles having three dimensional-like images using magnetic pigment compositions.
2. The Relevant Technology
Various pigments, colorants, and foils have been developed for a wide variety of applications. For example, magnetic pigments have been developed for use in applications such as decorative cookware, creating patterned surfaces, and security devices. Similarly, color shifting or optically variable pigments have been developed for such uses as cosmetics, inks, coating materials, ornaments, ceramics, automobile paints, anti-counterfeiting hot stamps, and anti-counterfeiting inks for security documents and currency.
Color shifting pigments, colorants, and foils exhibit the property of changing color upon variation of the angle of incident light, or as the viewing angle of the observer is shifted. The color-shifting properties of pigments and foils can be controlled through proper design of the optical thin films or orientation of the molecular species used to form parameters such as thickness of the layers forming the flakes and foils and the index of refraction of each layer. The changes in perceived color which occur for different viewing angles or angles of incident light are a result of a combination of selective absorption of the materials comprising the layers and wavelength dependent interference effects. The interference effects, which arise from the superposition of light waves that have undergone multiple reflections, are responsible for the shifts in color perceived with different angles. The reflection maxima changes in position and intensity, as the viewing angle changes, due to changing interference effects arising from light path length differences in the various layers of the optical stack.
Various approaches have been used to achieve such color shifting effects. For example, small multilayer flakes, typically composed of multiple layers of thin films, are dispersed throughout a medium such as paint or ink that may then be subsequently applied to the surface of an object. Such flakes may optionally be overcoated to achieve desired colors and optical effects. Another approach is to encapsulate small metallic or silicate substrates with varying layers and then disperse the encapsulated substrates throughout a medium such as paint or ink. Additionally, foils composed of multiple layers of thin films on a substrate material have been made.
One manner of producing a multilayer thin film structure is by forming it on a flexible web material with a release layer thereon. The various layers are deposited on the web by methods well known in the art of forming thin coating structures, such as PVD, sputtering, or the like. The multilayer thin film structure is then removed from the web material as thin film color shifting flakes, which can be added to a polymeric medium such as various pigment vehicles for use as an ink or paint. In addition to the color shifting flakes, additives can be added to the inks or paints to obtain desired color shifting results.
Color shifting pigments or foils are formed from a multilayer thin film structure that includes the same basic layers. These include an absorber layer(s), a dielectric layer(s), and optionally a reflector layer, in varying layer orders. One skilled in the art will recognize that an absorber layer is a semi-absorbing, semi transparent layer. The coatings can be formed to have a symmetrical multilayer thin film structure, such as:
absorber/dielectric/reflector/dielectric/absorber; or
absorber/dielectric/absorber.
Coatings can also be formed to have an asymmetrical multilayer thin film structure, such as:
absorber/dielectric/reflector.
For example, U.S. Pat. No. 5,135,812 to Phillips et al., which is incorporated by reference herein, discloses color-shifting thin film flakes having several different configurations of layers such as transparent dielectric and partially absorbing stacks. In U.S. Pat. No. 5,278,590 to Phillips et al., which is incorporated by reference herein, a symmetric three layer optical interference coating is disclosed which comprises first and second partially transmitting absorber layers which have essentially the same material and thickness, and a dielectric spacer layer located between the first and second absorber layers.
Color shifting platelets for use in paints are, disclosed in U.S. Pat. No. 5,571,624 to Phillips et al., which is incorporated by reference herein. These platelets are formed from a symmetrical multilayer thin film structure in which a first absorber layer such as chromium is formed on a substrate, with a first dielectric layer formed on the first absorber layer. A reflecting metal layer such as aluminum is formed on the first dielectric layer, followed by a second dielectric layer. A second absorber layer is formed on the second dielectric layer.
With regard to magnetic pigments, U.S. Pat. No. 4,838,648 to Phillips et al. (hereinafter xe2x80x9cPhillips ""648xe2x80x9d), the disclosure of which is incorporated by reference, describes a thin film magnetic color shifting structure in which a magnetic material can be used as a reflector or absorber layer in the structure. One disclosed magnetic material is a cobalt nickel alloy. Phillips ""648 discloses flakes and foils with the following structures:
dyed superstrate/absorber/dielectric/magnetic layer/substrate;
dyed superstrate/absorber/dielectric/magnetic layer/dielectric/absorber/dyed superstrate; and
adhesive/magnetic layer/dielectric/absorber/releasable hardcoat/substrate.
Patterned surfaces have been provided by exposing magnetic flakes to a magnetic force to effect a physical alteration in the structure of the pigment. For example, U.S. Pat. No. 6,103,361 to Batzar et al. (hereinafter xe2x80x9cBatzarxe2x80x9d) uses pigments made of magnetizable materials to decorate cookware. In particular, Batzar is directed toward controlling the orientation of stainless steel flakes in a fluoropolymer release coating to make patterns where at least some of the flakes are longer than the coating thickness. The patterned substrate is formed by applying magnetic force through the edges of a magnetizable die positioned under a coated base to alter the orientation of the flakes within the coating, thereby creating an imaging effect or pattern. However, Batzar does not discuss the use of optical thin film stacks or platelets employing a magnetic layer. In addition, although the stainless steel flakes used in Batzar are suitable for decorating cookware, they are poorly reflecting.
U.S. Pat. No. 2,570,856 to Pratt et al (hereinafter xe2x80x9cPrattxe2x80x9d) is directed to metallic flake pigments which are based on ferromagnetic metal platelets. Like Batzar, however, Pratt uses poorly reflecting metals and does not teach the use of thin film optical stacks.
U.S. Pat. No. 5,364,689 to Kashiwagi et al. and U.S. Pat. No 5,630,877, also to Kashiwagi, (hereinafter collectively xe2x80x9cKashiwagixe2x80x9d) disclose methods and apparatus for creating magnetically formed painted patterns. Kashiwagi uses a magnetic paint layer, which includes non-spherical magnetic particles in a paint medium, and applies a magnetic field with the magnetic field lines in the shape of the desired pattern. The final pattern is created by the different magnetic particle orientations in the hardened paint.
One attempt at incorporating a magnetic layer into a multilayer flake is disclosed in European Patent Publication EP 686675B1 to Schmid et al. (hereinafter xe2x80x9cSchmidxe2x80x9d), the disclosure of which is incorporated by reference. Schmid describes laminar color shifting structures which include a magnetic layer between the dielectric layer and a central aluminum layer as follows:
oxide/absorber/dielectric/magnet/Al/magnet/dielectric/absorber/oxide
Thus, Schmid uses aluminum platelets and then coats these platelets with magnetic materials. However, the overlying magnetic material downgrades the reflective properties of the pigment because aluminum is the second brightest metal (after silver), meaning any magnetic material is less reflective. Further, Schmid starts with aluminum platelets generated from ballmilling, a method which is limited in terms of the layer smoothness that can be achieved.
Patent Publication EP 710508A1 to Richter et al. (hereinafter xe2x80x9cRichterxe2x80x9d) discloses methods for providing three dimensional effects by drawing with magnetic tips. Richter describes three dimensional effects achieved by aligning magnetically active pigments in a spatially-varying magnetic field. Richter uses standard pigments (bariun ferrite, strontium ferrite, samarium/cobalt, Al/Co/Ni alloys, and metal oxides made by sintering and quick quenching, none of which are composed of optical thin film stacks. Rather, the particles are of the hard magnetic type. Richter uses electromagnetic pole pieces either on top of the coating or on both sides of the coating. However, Richter uses a moving system and requires xe2x80x9cdrawingxe2x80x9d of the image. This xe2x80x9cdrawingxe2x80x9d takes time and is not conducive to production type processes.
U.S. Pat. No. 3,791,864 to Steingroever (hereinafter xe2x80x9cSteingroeverxe2x80x9d) describes a method for patterning magnetic particles by orienting them with a magnetic pattern generated in an underlying prime coating that has previously been patterned by a magnetic field. The prime coat contains magnetic particles of the type xe2x80x9cMOxc3x976Fe2O3xe2x80x9d where M can be one or more of the elements Ba, Sr, Co, or Pb. After coating a continuous sheet of liquid coating of the primer, it is solidified and then areas of the primer are magnetized by a magnetic field. Next, a pigment vehicle with magnetic particles suspended therein is then applied. The magnetic particles suspended therein are finally oriented by the magnetic force from the magnetic pattern in the primer, creating the final pattern. However, Steingroever suffers from a diffuse magnetic image in the prime coat, which in turn passes a diffuse image to the topcoat. This reduction in resolution is because high magnetic fields are limited in the resolution they can create. This limitation is due to high magnetic field lines surrounding the intended magnetic image, thereby affecting untargeted magnetic particles in the prime coat and blurring the image.
Accordingly, there is a need for improved methods and devices that overcome or avoid the above problems and limitations.
It is an object of the invention to provide printed images with three-dimensional like features.
It is another object of the invention to provide imaged coating structures with covert and/or overt security features.
It is yet another object of the invention to provide methods to fabricate magnetic printed images that are compatible with mass production.
To achieve the foregoing objects and in accordance with the invention as embodied and broadly described herein, methods and devices for fabricating three dimensional-like images on coated articles are provided. The methods generally comprise applying a layer of magnetizable pigment coating in liquid form on a substrate, with the magnetizable pigment coating containing a plurality of magnetic non-spherical particles or flakes. A magnetic field is exposed to selected regions of the pigment coating while the coating is in liquid form, with the magnetic field altering the orientation of selected magnetic particles or flakes. Finally, the pigment coating is solidified, affixing the reoriented particles or flakes in a non-parallel (or intermediate) position to the surface of the pigment coating to produce an image or a three dimensional-like image on the surface of the coating. The pigment coating can contain various interference or non-interference magnetic particles or flakes, including magnetic color shifting pigments.
In one method of the invention, the magnetic field is provided by a magnetic printed image layer applied to the substrate prior to applying the pigment coating. In another method, an external magnetic source is utilized such as a sheet magnet configured in the shape of the desired image, with the sheet magnet placed on the surface of the substrate opposite the pigment coating. Other external magnetic sources that can be used include a DC magnetron sputtering magnetic cathode, a magnetizable die selectively magnetized by a secondary magnetic source, or multiple magnetic poles oriented together to from an image below the substrate.
A preferred pigment composition for use with the present methods includes multilayer magnetic thin film flakes or particles, which can have a symmetrical stacked coating structure on opposing sides of a magnetic core layer, or can be formed with one or more encapsulating coatings around a magnetic or dielectric core. Optional additional layers, such as dielectric and absorber layers, can be added to overlie the flakes or particles and thereby add a color shift or other features to the pigments. Suitable pigment flakes include those that have a magnetic layer between reflector or dielectric layers, a dielectric layer between magnetic layers, or monolithic magnetic layers.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.